[CSS Blending] Implement mix-blend-mode in software.

Source/core/rendering/RenderLayer.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 811 matching lines @@
     void clearZOrderLists();
     void setIsUnclippedDescendant(bool isUnclippedDescendant) { m_isUnclippedDescendant = isUnclippedDescendant; }
 
 
     void updateNormalFlowList();
 
     bool isStackingContext(const RenderStyle* style) const { return !style->hasAutoZIndex() || isRootLayer(); }
 
     bool isDirtyStackingContainer() const { return m_zOrderListsDirty && isStackingContainer(); }
 
+    bool hasNonAcceleratedBlendedChildInCurrentStackingContext() const;
+
     void setAncestorChainHasSelfPaintingLayerDescendant();
     void dirtyAncestorChainHasSelfPaintingLayerDescendantStatus();
 
     void setAncestorChainHasOutOfFlowPositionedDescendant();
     void dirtyAncestorChainHasOutOfFlowPositionedDescendantStatus();
 
     bool acceleratedCompositingForOverflowScrollEnabled() const;
     // FIXME: This is a temporary flag and should be removed once accelerated
     // overflow scroll is ready (crbug.com/254111).
     bool compositorDrivenAcceleratedScrollingEnabled() const;
@@ skipping 298 matching lines @@
     // descendants in stacking order. This is one of the requirements of being
     // able to safely become a stacking context.
     unsigned m_canBePromotedToStackingContainer : 1;
     unsigned m_canBePromotedToStackingContainerDirty : 1;
 
     const unsigned m_isRootLayer : 1;
 
     unsigned m_usedTransparency : 1; // Tracks whether we need to close a transparent layer, i.e., whether
                                  // we ended up painting this layer or any descendants (and therefore need to
                                  // blend).
+    unsigned m_isolateLayerFromBlendedContent : 1; // Blending through the mix-blend-mode operation should
+        // only be performed with the contents of the underlying

[shawnsingh, 2013/09/19 11:05:56]

Some nitpicks here - 

(1) Is it correct to say "A layer with a mix-blend-mode should only be blended
with content underneath this layer, within the same stacking context" ?  I think
that would be more clear.   The word "underlying" here (and actually, also in
the spec) feels a bit unclear to me.

(2) In my opinion, the name "m_isolateForDescendantBlendMode" might be clearer
here. 

[mitica, 2013/09/19 11:49:00]

Sounds good, I'll change it.

+        // stacking context.
     unsigned m_paintingInsideReflection : 1; // A state bit tracking if we are painting inside a replica.
     unsigned m_inOverflowRelayout : 1;
     unsigned m_repaintStatus : 2; // RepaintStatus
 
     unsigned m_visibleContentStatusDirty : 1;
     unsigned m_hasVisibleContent : 1;
     unsigned m_visibleDescendantStatusDirty : 1;
     unsigned m_hasVisibleDescendant : 1;
 
     unsigned m_isPaginated : 1; // If we think this layer is split by a multi-column ancestor, then this bit will be set.
@@ skipping 148 matching lines @@
 
 } // namespace WebCore
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showLayerTree(const WebCore::RenderLayer*);
 void showLayerTree(const WebCore::RenderObject*);
 #endif
 
 #endif // RenderLayer_h